gigaset: CAPI module readability improvements

Replace the "ignoring Additional Info" warning message by better
readable ones citing the specific subparameters being ignored.
Make parts of the code more readable by using a local cmsg
pointer variable.

Impact: readability improvement
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>